The present invention relates generally electroluminescent devices and more specifically relates to an organic electroluminescent device having a thin film optical interference layer to reduce reflectance from ambient light.
Electroluminescent devices (ELDs) are well known and are generally constructed of several layers of different materials. These layers typically consist of a transparent front-electrode layer, an electroluminescent layer and a back-electrode layer. When a voltage is applied across the electrodes, the electroluminescent layer becomes active, converting some portion of the electrical energy passing therethrough into light. This light is then emitted out through the front-electrode, which is transparent to the emitted light, where it is visible to a user of the device.
Electroluminescent devices can be particularly useful as computer displays and are generally recognized as high-quality displays for computers and other electronic devices used in demanding applications such as military, avionics and aerospace where features such as high reliability, low weight, and low power consumption are important. Electroluminescent displays are also gaining recognition for their qualities in automotive, personal computer and other consumer industries, as they can offer certain benefits over other displays such as cathode-ray tubes (xe2x80x9cCRTxe2x80x9d) and liquid crystal displays (xe2x80x9cLCDxe2x80x9d).
One feature of electroluminescent displays is the ability to add thin films between the layers to vary the characteristics of the display. It is known to use thin film layers in electroluminescent displays to improve selected display characteristics, such as signal-to-reflected-ambient light ratio (xe2x80x9cSRAxe2x80x9d) and contrast ratio (xe2x80x9cCRxe2x80x9d). For greater clarity, signal-to-reflected ambient light ratio can be defined as:   SRA  =            L      em              R      xc3x97              L        amb            
where:
SRA=Signal-to-reflected ambient light ratio
Lem=Emitted luminance of the device
R=Reflectance of the device
Lamb=The ambient illuminance, or the ambient light incident on the display
and, in a pixilated device, contrast ratio can be defined as:   CR  =                    L        on            +              R        xc3x97                  L          amb                                    L        off            +              R        xc3x97                  L          amb                    
where:
CR=Contrast Ratio
Lon=Emitted luminance of active or xe2x80x9conxe2x80x9d pixels
Loff=Emitted luminance of inactive or xe2x80x9coffxe2x80x9d pixels
R=Reflectance of the device
Lamb=The ambient illuminance, or the ambient light incident on the display
One particular type of thin-film layer that can be used to improve contrast ratio in electroluminescent devices is a substantially transparent optical interference layer placed between one or more of the layers of the electroluminescent device, as taught in U.S. Pat. No. 5,049,780 to Dobrowolski. As will be apparent to those of skill in the art, improvements to the contrast ratio of an electroluminescent device is generally desirable and particularly important in avionics and military applications where poor contrast and glare can have serious consequences. Using the principle of destructive interference, the optical interference layer can result in the reduction of the amplitude of ambient light by superimposing of two or more, out-of-phase, electromagnetic waves, which can be generated by reflection and/or transmission at the interfaces of thin-film layer(s). By selecting appropriate thicknesses of the layers, optical destructive interference at the electromagnetic wavelengths of interest (typically visible ambient light waves reflected off of the display) can result in an exceptional contrast ratio and/or signal-to-reflected ambient light ratio.
Dobrowolski is generally directed to voltage-driven inorganic electroluminescent devices, where the electroluminescent layer is formed of an inorganic material, and which typically require one or more additional transparent dielectric layers to reduce electrical-breakdown of the inorganic electroluminescent layer. Such inorganic electroluminescent devices are typically voltage-driven, powered with alternating current (xe2x80x9cacxe2x80x9d) in order to reduce charge build-up within the device. While Dobrowolski does generally contemplate the use of direct current (xe2x80x9cdcxe2x80x9d) electroluminescent devices without transparent dielectric layers, such inorganic devices are still voltage-driven, and are generally prone to electrical breakdown of the electroluminescent layer.
With the advent of modem current-driven organic electroluminescent devices which offer certain advantages (such as colour improvements and a reduced barrier to current flow to reduce the necessary drive voltage) compared to voltage-driven inorganic electroluminescent devices, there is now a need to improve the contrast ratio and/or signal to ambient light ratio of these organic devices, and it can be seen that the prior art does not teach a suitable optical interference electroluminescent device to address this need.
It is therefore an object of the present invention to provide a novel optical interference organic electroluminescent device which obviates or mitigates at least one of the disadvantages of the prior art.
In an embodiment of the invention there is provided an optical interference electroluminescent device for displaying an image to a viewer in front of the device, comprising: an anode layer; a cathode layer, at least one of the anode layer and the cathode layer being substantially transparent to at least a portion of emitted electroluminescent light; at least one organic electroluminescent layer disposed between the anode layer and the cathode layer, the electroluminescent layer having a first energy characteristic being the amount of energy required to extract an electron from a highest occupied molecular orbital of the electroluminescent layer, and a second energy characteristic being the amount of energy required to extract an electron from a lowest unoccupied molecular orbital of the electroluminescent layer; and at least one optical interference member disposed between two of the layers, and having a work function substantially equal to the first energy characteristic when the optical interference member is between the anode and the electroluminescent layer, and having a work function substantially equal to the second energy characteristic when the optical interference member is between the cathode and the electroluminescent layer, the optical interference member being of a thickness and material such that the spectral reflectance of the electroluminescent device is so modified that the reflectance of ambient light by the electroluminescent device towards the viewer is reduced.
In another embodiment of the invention, there is provided an electroluminescent device to emit light in a selected spectrum, comprising: an anode layer, a cathode layer, wherien one of the anode layer and the cathode layer are substantially transparent to at least a portion of the selected spectrum emitted by the electroluminescent device. The electroluminescent device further comprises an organic electroluminescent layer between the anode layer and the cathode layer, the electroluminescent layer having a highest occupied molecular orbital respective to the anode layer and having a lowest unoccupied molecular orbital respective to the cathode layer. The device further includes an optical interference member having a selected work function and operable to reduce ambient light reflected through the transparent layer, the optical interference member being between the electroluminescent layer and one of the anode layer and the cathode layer, wherein the difference between the selected work function and an energy level required to extract an electron from a respective molecular orbital approaches zero.
In another embodiment of the invention, there is provided a method of fabricating an electroluminescent device for displaying an image to a viewer in front of the device, comprising the steps of:
depositing an anode layer onto a substrate;
depositing an organic electroluminescent layer onto the anode layer, the electroluminescent layer having a first energy characteristic associated with an anode side, and a second energy characteristic associated with a cathode side;
depositing an optical interference member onto the electroluminescent layer, the optical interference member for reducing the reflectance of ambient light towards the viewer, the optical interference member having a work function substantially equal to the second energy characteristic;
depositing a cathode layer onto the optical interference member; and sealing the device.
In another embodiment of the invention there is provided a method of assembling an electroluminescent device for displaying an image to a viewer in front of the device, comprising the steps of:
depositing an anode layer onto a substrate;
depositing an optical interference member onto the anode layer; the optical interference member for reducing the reflectance of ambient light towards the viewer, the optical interference member having a work function;
depositing an organic electroluminescent layer onto the anode layer, the electroluminescent layer having an energy characteristic being the amount of energy required to extract an electron from the electroluminescent layer, the energy characteristic being substantially equal to the work function;
depositing a cathode layer onto the electroluminescent layer; and sealing the device.
In another embodiment of the invention, there is provided a method of displaying an image to a viewer comprising the steps of:
emitting light from an organic electroluminescent layer between an anode and a cathode, said electroluminescent layer having a first energy characteristic respective to said anode and a second energy characteristic respective to said cathode; and
receiving ambient light incident towards said electroluminescent layer; and
forming destructive interference from said ambient light at the incident surface of an optical interference member, said optical interference member having a selected work function and disposed between said electroluminescent layer and one of said anode and said cathode, the difference between said work function and a respective energy characteristic approaching zero.
The appropriate selection of material of the at least one optical interference member ensures proper current flow through the device, thus reducing the likelihood of electrical breakdown of the organic electroluminescent layer, and improving the overall energy efficiency of the device, while still reducing reflectance towards a viewer.